An altimeter is a device used to measure altitude, or the height above a specific reference point, usually sea level. It works by detecting changes in air pressure; as altitude increases, air pressure decreases. There are two main types of altimeters: barometric altimeters, which rely on atmospheric pressure, and GPS altimeters, which use satellite signals to determine position and height. Altimeters are commonly found in aircraft, where they help pilots maintain safe flying altitudes. They are also used in outdoor activities like hiking and climbing, allowing adventurers to track their elevation changes. Aircraft, GPS, and hiking are all related to the use of altimeters.